Several lines of evidence suggest that the hormone melatonin may inhibit breast cancer. However, to date no epidemiologic studies of breast cancer have included assessment of melatonin levels. The overall objective of this study is to determine how to reliably measure melatonin levels for use in epidemiologic studies. A nocturnal/ morning void urine sample and 4 daytime spot urine samples throughout the day will be obtained from 50 women diagnosed with invasive breast cancer and 50 women with no prior history of breast cancer. In addition, a second set of urine samples will be obtained from a subsample of 20 women (10 cases and 10 controls) approximately one month after the first collection in order to assess intra- individual variation. Cases will be enrolled after completion of treatment. Levels of 6-sulfatoxymelatonin (6-SMT), the major metabolite of melatonin in urine, will be determined by a highly sensitive and reliable radio-immunoassay, and creatinine will be assayed. Questionnaire data will also be collected on factors thought to influence melatonin levels and on breast cancer risk factors. A number of parameters will be used to characterize melatonin excretion patterns, including the proportion of women with a peak during the nocturnal hours, mean nocturnal level, and the amplitude (maximum - minimum 6-SMT). The specific aims are: l) to estimate the proportion with a nocturnal peak, the mean nocturnal level, and the mean amplitude among cases and controls and to determine whether these parameters differ between the two groups; 2) to assess intra-individual variation in 6-SMT levels by resampling a subsample of women; 3) to identify predictors of 6-SMT parameters (including sleep hours, alcohol intake, body mass index, and other variables); and 4) to assess the prediction of peak nocturnal levels from a daytime spot urine sample. This project will provide valuable information for the design of epidemiologic studies investigating the relationship of melatonin to breast and other cancers. If a significant proportion of women have their peak outside the nocturnal period, this would argue for the necessity of collecting additional urine samples throughout the day.